Device for preventing soiling of the trailing end portion of a transfer sheet

ABSTRACT

A device for use with electrostatic latent image transferprinting apparatus to prevent soiling of the trailing end portion of the transfer-printing sheet due to overcharging, which device is disposed in a position to impress a bias voltage of the same charge as the electrostatic latent image on the electricallyconducting material layer underside of the transfer-printing sheet immediately after the latent-image containing dielectric upper layer is stripped from the surface of the image forming member.

United States Patent [191 Adachi et al. June 18, 1974 [5 1 DEVICE FOR PREVENTING SOILING 0F 3,240,596 3/1966 Medley et al. 96/1 R THE TRAILING END PORTION OF A 3,345,925 10/1967 Ostensen 355/ 10 Sage Inventors: Tsukasa Adachi, Tokyo; Kazuhiko Kasuya, Kawasaki, both of Japan Assignee: Ricoh Co., Ltd., Tokyo, Japan Filed: Dec. 18, 1972 Appl. No.: 316,100

Foreign Application Priority Data Dec. 28, 1971 Japan 46-364 US. Cl. 355/3 R, 355/16, 96/1 R Int. Cl G03g 15/00 Field of Search 355/3, 10, 17, 8, 16;

References Cited UNITED STATES PATENTS Schaffert 96/1 R Primary Examiner-Robert P. Greiner Attorney, Agent, or FirmCooper, Dunham, Clark, Griffin Moran A device for use with electrostatic latent image transfer-printing apparatus to prevent soiling of the trailing end portion of the transfer-printing sheet due to overcharging, which device is disposed in a position to impress a bias voltage of the same charge as the electrostatic latent image on the electrically-conducting material layer underside of the transfer-printing sheet immediately after the latent-image containing dielectric ABSTRACT I upper layer is stripped from the surface of the image forming member.

6 Claims, 5 Drawing Figures PATENTEDJUII 18 I974 SHEET 10F 3 F I G. {PRIOR ART F|G.2PRIOR ART PATENTEBM sum 2 or 3 F l' 6.3 PRIOR ART DEVICE FOR PREVENTING SOILING OF THE TRAILING END PORTION OF A TRANSFER SHEET BACKGROUND OF THE INVENTION This invention relates to a device for preventing soiling of the'trailing end portion of a transfer-printing sheet due to overcharging.

In one type of electrostatic latent image transferprinting method known in the art, a photosensitive drum serving as an image forming member is produced by providing a photoconductive material layer on a drum made of metal or other electrically conducting material, and an electrostatic latent image is formed on the photoconductive material layer of the photosensitive drum by charging and exposing the same to an optical image of an original. On the other hand, a dielectric layer is provided on an electrically conducting material layer to provide a transfer-printing sheet which is brought into intimate contact with the electrostatic latent image bearing surface of the photosensitive drum and then stripped off therefrom so as to effect transferprinting of the electrostatic latent image on the transfer-printing sheet.

The electrostatic latent image transfer-printing method described has a disadvantage in that, if the electrically conducting material layer forming a part of the transfer-printing sheet has a relatively high resistance value or if the transfer-printing sheet is fed at high speed in order to increase efficiency by effecting transfer-printing at high speed, the trailing end portion of the transfer-printing sheet gets soiled for an extent of about several to several scores of millimeters due to adhesion of the toner in a greater quantity'than is necessary when the transfer-printed electrostatic latent imageis developed. The image forming member may be in any other form, such for example as in sheet form, and need not be in drum form as described.

One type of prior art apparatus for carrying the aforementioned electrostatic latent image transferprinting method into practice will be described in connection with FIG. 1-3. In FIG. 1, a photosensitive drum 1 is produced by providing an electrically conducting drum 2 made of aluminum, copper or the like with a photoconductive material layer 3 by coating its peripheral surface with selenium, zinc oxide, poly-N-vinyl carbazole or other photoconductive material. Photosensitive drum 1 is supported by a shaft 4 for rotation in the direction of an arrow a.

The peripheral surface of photosensitive drum 1 is electrically charged by means of a charger 8 comprising a wire electrode 7 on which a high bias voltage is impressed by a power source 6, and exposed by slit exposing means 13 comprising a projection lens 9, reflector l and slit member 11 formed with a slit S to an optical image of an original 14 moving on an original support surface 13 in synchronizm with the rotation of photosensitive drum 1.

A transfer-printing roller 15 made as of electrically conducting rubber produced by incorporating carbon black or the like in rubber is maintained in light contact with a lower portion of the periphery of photosensitive drum 1. A transfer sheet 16 is fed between the peripheral surfaces of photosensitive drum 1 and transferprinting roller 15 by a pair of feed rollers 17. Transferprinting sheet 6 comprises an electrically conducting material layer 16a and a dielectric layer 16b made of polyvinyl chloride, a copolymer of vinyl chloride and vinyl acetate or the like and disposed above electrically conducting material layer as shown in FIG. 2.

Upon being fed by the pair of feed rollers 17 between photosensitive drum 1 and roller 15, transfer-printing sheet 16 has its dielectric layer 16b brought into intimate contact with the electrostatic latent image on the peripheral surface of drum 1 so that the latent image on the drum periphery may be transferred to the dielectric layer of transfer-printing sheet 16, and then sheet 16 is stripped off from the drum periphery by a stripper 18 and conveyed to a developing device 20 by a pair of conveyor rollers 19 comprising rollers 19a and 19b.

Developing device 20 as shown comprises a developing liquid tank 21 containing therein a developing liquid 22 and having upper and lower fins 23 and 24, respectively, provided in the developing liquid for guiding transfer-printing sheet 16 in its movement through the developing liquid 22 in developing liquid tank 21 to render the electrostatic latent image thereon visible. After having its electrostatic latent image developed into a visible image, transfer-printing sheet 16 is discharged from tank 21 by a pair of squeeze and conveyor rollers 25. The developing device may be of other forms than the device shown, such for example as other types of developing device of the wet developer type or a developing device of any known dry developer type.

The present practice is to impress a voltage of an opposite charge to the electrostatic latent image on transfer roller 15 by a power source 26 or to ground transfer-printing sheet 15 to achieve satisfactory results in effecting transfer-printing. An attempt is made to achieve electric balancing of transfer-printing sheet 16 by using a roller of an electrically conducting material as one roller 19b of the pair of rollers 19 and grounding it so that the electrically conducting material layer 16a of transfer-printing sheet 16 in contact with it is grounded.

' Experiments conducted to ascertain the electric conditions of transfer-printing sheet 16 have produced the following results:

1. When the resistance velue of electrically conducting material layer 16a of transfer-printing sheet 16 is low, the trailing end of transfer-printing sheet 16 does not get soiled;

2. When the resistance value of the electrically conducting material layer 16a is high, not only the trailing end but also the entire surface of transferprinting sheet 16 gets soiled;

3. When transfer-printing roller 15 is disposed nearer to the stripper 18, transfer-printing sheet 16 is hard to soil; and

4. By impressing a bias voltage (several scores to several hundreds of volts) of the same charge as the electrostatic latent image on the roller 19b of the pair of conveyor rollers 19 which is disposed anterior to the transfer-printing roller 15 and in contact with the electrically conducting material layer 16a of transfer-printing sheet 16, it is possible to prevent soiling of the trailing end portion of transferprinting sheet 16.

SUMMARY OF THE INVENTION This invention has as its object the provision of a device for preventing soiling of the trailing end portion of a transfer-printing sheet due to overcharging by impressing a bias voltage, of the same charge as an electrostatic. latent image formed on an image fomiing member, on a portion of an electrically conducting material layer of the transfer-printing sheet disposed near a position in which the dielectric layer of the transferprinting sheet is stripped off from the dielectric layer of the image forming member.

According to the invention, a voltage of the same .charge as the electrostatic latent image formed on the image forming member is impressed on the portion of the electrically conducting material layer of the transfer-printing sheet which is disposed in a position in which the transfer-printing sheet is released from engagement with the image forming member and a transfer-printing roller or near to a stripper, so that overcharging of the trailing end portion of the transferprinting sheet can be prevented at all times and soiling thereof can be prevented.

BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF A PREFERRED EMBODIMENT The mechanism of soiling of the trailing end portion of a transfer-printing sheet will be discussed first. While transfer-printing sheet 16 isbeing maintained in contact with both transfer-printing roller and conveyor roller 19b as shown in FIG. 2, the electrically conducting material layer 16a of transfer-printing sheet 16 is at a relatively low potential. However, when the trailing end of transfer-printing sheet 16 is released from engagement with transfer-printing roller 15 as shown in FIG. 3, the portion of the electrically conducting material layer disposed at the trailing end portion of transfer sheet 16 is charged oppositely to the electrostatic latent image formed on the dielectric layer 16b in a larger amount than is consistent with the electric potential at the electrostatic latent image due to the influence of a residual potential on photo-sensitive drum 1. The amount of the charge of opposite sign to the electrostatic latent image is greater when the speed at which the transfer-printing sheet is released from engagement with the transfer-printing roller is higher. Thus, the trailing end portion of the electrically conducting material layer of the transfer-printing sheet is excessively charged and performs the role of an opposite electrode when the latent image is developed, so that it promotes adhesion of toner to the dielectric layer 16b and causes soiling to occur.

This phenomenon may be considered as follows: When the transfer-printing sheet is stripped off from the photosensitive drum, a strip off current i flows which is of an opposite charge to a transfer-printing current which flows when transfer-printing is effected, and a potential V at a portion of the transfer-sheet which is stripped off from the photosensitive drum may be expressed as V= iR where R is the resistance of the electrically conducting material layer. The potential V acts as a bias voltage when the electrostatic latent image is transferredfrom the photosensitive drum to the transfer-printing sheet. I

It has been'found that, assuming that i is constant, the following means may be provided to reduce the value of potential V: v

1. To reduce the value of resistance R; v

2. To decrease the spacing between the transferprinting'roller and the stripper; or

3. To impress a bias voltage VB so that potential V can be expressed as V iR-VB.

The present invention is based on the aforementioned discovery. According to the invention, soiling of the trailing end portion of the transfer-printing sheet can beprevented by impressing a voltage of the same charge as the electrostatic latent image on the portion of the electrically conducting material layer of the transfer-printing sheet which is disposed in a position in which the transfer-printing sheet is released from engagement with the photosensitive drum and the transfer printing roller.

FIG. 4 shows one embodiment of the invention which comprises voltage impressing means comprising a power source 27 and which is incorporated in the apparatus shown in FIG. 1 for impressing a voltage in a range of several scores to several hundreds of volts of the same charge as the electrostatic latent image on conveyor roller 19b, so as to prevent soiling of the trailing end portion of the transfer-printing sheet.

According to the invention, a bias voltage may be impressed on a guide plate 28 (see FIG. 5) of an electrically conducting material disposed in the vicinity of transfer-printing roller 15 in place of on conveyor roller 19b. Other well-known means than the means described herein may be used as the voltage impressing means.

Preferably, electrically conducting conveyor roller 19b is made of electrically conducting rubber or metal. The invention can have application in a transferprinting method in which a dielectric layer is provided on the electrically conducting drum for forming thereon an electrostatic latent image as by means of a contact electrode, in place of using photosensitive drum 1. The photosensitive drum or the aforementioned drum formed with the dielectric layer may be in sheet form in place of drum form. The electrically conducting material layer may be made, not of metal, but of a sheet of paper or plastic treated to render it electrically conducting by applying electrically conducting rubber to its surface or laminating an aluminum sheet to it or a sheet of paper made by incorporating an electrically conducting material paper stock.

What is claimed is:

1. In an electrophotographic reproduction device of the transfer-printing type in which an electrostatic latent image is formed on the dielectric surface of an image forming member having a dielectric layer on an electrically conducting material layer, and the dielectric surface of a transfer-printing sheet having a dielectric layer provided on an electrically conducting material layer is brought into intimate contact with the electrostatic latent image bearing surface of the image forming member and then stripped off to effect transfer-printing of the electrostatic latent image on the transfer-printing sheet, the improvement comprising voltage impressing means including a power source for impressing a bias voltage of the same charge as the 3,817,615 6 electrostatic latent image on a portion of the electri- 3. A device as set forth in claim 1 in which said voltcally conducting material layer of the transfer-printing age impressing means comprises a transfer-printing sheet, which voltage impressing means is disposed in a sheet guide plate of an electrically conducting material. position immediately beyond that at which the dielectric layer of the transfer-printing sheet is stripped off 5 4. A device as set forth in claim 2 wherein said transfrom the dielectric layer of the image forming member fer-printing sheet conveying electrically conducting whereby soiling of the trailing end portion of the transroller is made of metal.

fer-printing sheet due to overcharging can be pre- 5. A device as set forth in claim 2 in which said transvented. fer-printing sheet conveying electrically conducting 2. A device as set forth in claim 1 in which said volt- 1Q roller is made of electrically conducting rubber. age impressing means comprises a transfer-printing 6. A device as set forth in claim 3 in which said transsheet conveying electrically conducting roller confer-printing sheet guide plate is made of metal. nected to said power source. 

1. In an electrophotographic reproduction device of the transfer-printing type in which an electrostatic latent image is formed on the dielectric surface of an image forming member having a dielectric layer on an electrically conducting material layer, and the dielectric surface of a transfer-printing sheet having a dielectric layer provided on an electrically conducting material layer is brought into intimate contact with the electrostatic latent image bearing surface of the image forming member and then stripped off to effect transfer-printing of the electrostatic latent image on the transfer-printing sheet, the improvement comprising voltage impressing means including a power source for impressing a bias voltage of the same charge as the electrostatic latent image on a portion of the electrically conducting material layer of the transfer-printing sheet, which voltage impressing means is disposed in a position immediately beyond that at which the dielectric layer of the transferprinting sheet is stripped off from the dielectric layer of the image forming member whereby soiling of the trailing end portion of the transfer-printing sheet due to overcharging can be prevented.
 2. A device as set forth in claim 1 in which said voltage impressing means comprises a transfer-printing sheet conveying electrically conducting roller connected to said power source.
 3. A device as set forth in claim 1 in which said voltage impressing means comprises a transfer-printing sheet guide plate of an electrically conducting material.
 4. A device as set forth in claim 2 wherein said transfer-printing sheet conveying electrically conducting roller is made of metal.
 5. A device as set forth in claim 2 in which said transfer-printing sheet conveying electrically conducting roller is made of electrically conducting rubber.
 6. A device as set forth in claim 3 in which said transfer-printing sheet guide plate is made of metal. 